Should I get the Moon 891 streamer/dac/preamp for my Moon 861 amp? Or separates?

well the obvious underlying factor here is the different spaces. Your room isn’t the store. Your room dimensions differ from the store. Your furnishing is different. The distance you listen at  in your room is surely different from where you sat in the store demo

As you can hear deficiencies in bass and some bloat. it is a room related issue. Basically room modes differ in different rooms. Someone’s null might be your bloat. and you can’t tackle this by changing preamp. That is an impossibility. You need to tackle room/speaker coupling, seating distance (so you’re not sitting in nasty bass nulls)

If you continue to fly blind. Yes you may buy the Moon Audio but a different Preamp doesn’t solve room nulls (that is an impossibility. Physics can’t be whished away with purchases)

So, you’re better off  knowing the approximate tonal identity your room in imparting under 500Hz in your room first. What are your room dimensions, listening distance and distances of speaker from front wall and side walls? I can simulate something in REW @dman777 

DAC will make a huge difference in that system. Let’s start with what you can audition. What’s available from your local dealers or dealers that can send you a component for auditioning in your system. Don’t blind buy. You will get on a merry go round and the ride will suck.
Do you have access to dealers that cary Grimm, Mola Mola, Meitner, Bricasti, dCS? These are some of the brands that feature excellent one piece DAC with built in streamer and preamp options. 

@audphile1 But don't I need to audition the DAC and streamer on the same equipment? What if the store has a dac and streamer I am interested in but does not carry Moon or Focal?

@dman777 - the listening environment/room will likely make a lot more difference in sound than the gear. Something that sounds great in the showroom might sound terrible in your listening room, and vice-versa. 

Do you have a dealer who will let you audition at home?
 

I have owned McIntosh and when I decided to upgrade I auditioned Moon and Audio Research. Ultimately I went t with Moon as I didn’t want to deal with tubes. 

I prefer combined DAC / Streamer - personal preference and there are trade offs for either approach  

The question I believe you are trying to answer is would you prefer another component(s) in lieu of the Moon 891.  Good luck in your journey- I would look at DCS and Linn based on my experience and the guess digital is your primary music source.